A conventional hydraulic drive system for construction machines comprises, as disclosed in JP, A, 1-25921, a hydraulic pump of variable displacement type, a pump regulator for controlling a delivery rate of the hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid supplied from the hydraulic pump, a plurality of directional control valves of center bypass type for controlling respective flows of the hydraulic fluid supplied from the hydraulic pump to the plural hydraulic actuators, a center bypass line for connecting in series center bypasses of the plural directional control valves to a reservoir, a flow resistive element, e.g., a fixed restrictor, disposed in a downstream portion of the center bypass line for producing a control pressure, a pressure sensor for detecting the control pressure produced by the fixed restrictor and outputting a corresponding electric signal, and a function generator for storing preset one kind of pump flow rate characteristic that defines the relationship between a value of the electric signal outputted from the pressure sensor and a delivery rate of the hydraulic pump, determining the delivery rate corresponding to the value of the electric signal outputted from the pressure sensor based on the preset pump flow rate characteristic, and outputting, as a drive signal, a signal corresponding to the determined delivery rate. The pump regulator is driven with the drive signal.
In the above prior art, a variable restrictor for bleeding-off is disposed in the center bypass of each of the plural directional control valves. This variable restrictor is fully opened when the associated directional control valve is at its neutral position, and its opening is reduced as an input amount of the directional control valve increases. As a result, with the directional control valve being at its neutral position, the flow rate of the hydraulic fluid passing through the center bypass is maximized and, therefore, the control pressure produced by the fixed restrictor is also maximized. Then, as the input amount of the directional control valve increases, the flow rate through the center bypass is reduced and so is the control pressure. The pump flow rate characteristic preset in the function generator is set such that the delivery rate of the hydraulic pump is increased with the control pressure becoming smaller. Accordingly, the delivery rate of the hydraulic pump is controlled to increase dependent upon the input amount of the directional control valve.
Meanwhile, there are various kinds of operations to be performed by construction machines such as hydraulic excavators, and the directional control valve requires control characteristics different from each other dependent upon the kinds of operations. For example, in the work such as craning which requires fine operation, a control characteristic superior in the metering property is needed. On the other hand, in the work such as digging which requires powerful operation, a control characteristic superior in rising of the metering property and capable of easily supplying the hydraulic fluid at a large rate is needed.
In the conventional hydraulic drive system as stated above, however, the control characteristic of the delivery rate of the hydraulic pump is uniquely determined dependent upon the setting in the function generator and, correspondingly, the control characteristic of the directional control valve is also uniquely determined. This has raised the problem that good operating efficiency cannot be ensured in other kinds of work than particular one.
More specifically, in the above-explained prior hydraulic drive system, the control characteristic of the directional control valve is determined dependent upon the setting of the function generator as follows. When one directional control valve is operated, for example, the delivery rate of the hydraulic pump is controlled dependent upon the setting of the function generator as mentioned above, and the hydraulic fluid is supplied to the directional control valve at the controlled flow rate. The directional control valve supplies the hydraulic fluid to the actuator therethrough at a flow rate resulted by subtracting, from the pump delivery rate, the flow rate of the hydraulic fluid flowing out through the bleeding-off variable restrictor (i.e., the flow rate through the center bypass), dependent upon the opening area of the bleeding-off variable restrictor which is determined by the input amount (i.e., stroke) of the directional control valve at that time. In this case, because the control characteristic of the delivery rate of the hydraulic pump with respect to the valve stroke is fixed and the opening characteristic of the bleeding-off variable restrictor with respect to the valve stroke is also fixed, the control characteristic of the directional control valve, such as a metering characteristic, with respect to the flow rate of the hydraulic fluid supplied to the actuator becomes fixed.
Accordingly, when the pump flow rate characteristic preset in the function generator is set to give a control characteristic suitable for the work such as digging, for example, which requires powerful operation, it is difficult to perform fine operation in the work such as craning, for example, which requires fine operation. On the contrary, when the pump flow rate characteristic preset in the function generator is set to give a control characteristic suitable for the work such as craning, for example, which requires fine operation, the machine operates too slow to efficiently perform the work such as digging, for example, which requires powerful operation.
An object of the present invention is to provide a hydraulic drive system for construction machines in which the flow rate characteristic of a hydraulic pump can be changed to make the control characteristic of a directional control valve variable, thereby ensuring good operating efficiency for plural different kinds of work.